Calibration of quadrature imbalance via loopback phase shifts

ABSTRACT

Apparatuses, systems, and methods for calibration of quadrature imbalance in direct conversion transceivers are contemplated. A transceiver controller may perform a self-calibration to address quadrature imbalance. The controller may isolate the transmitter and receiver from any antennas, couple the radio frequency (RF) section of the transmitter to the RF section of the receiver via a loopback path, and inject a calibration signal into the transmitter. In the loopback path, the controller may phase-shift the signal that propagates through the transmitter using two different phase angles to produce two different signals that propagate into the receiver. By measuring the two different signals that exit the receiver, the controller may be able to calculate correction coefficients, or parameters, which may be used to adjust elements that address or correct the quadrature imbalance for both the transmitter and receiver.

FIELD

The embodiments herein are in the field of communications. Moreparticularly, the embodiments relate to methods, apparatuses, andsystems for calibrating quadrature imbalance in direct conversiontransceivers.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the embodiments will become apparent upon reading thefollowing detailed description and upon reference to the accompanyingdrawings in which like references may indicate similar elements:

FIG. 1 depicts a direct conversion transceiver;

FIG. 2 illustrates how a direct conversion transceiver may calibratequadrature imbalance in one embodiment;

FIG. 3 depicts a correction module that may be used in an alternativeembodiment;

FIG. 4 depicts one embodiment of an apparatus that may calibratequadrature imbalance in a direct conversion transceiver; and

FIG. 5 illustrates a method for calibrating quadrature imbalance indirect conversion transceivers.

DETAILED DESCRIPTION OF EMBODIMENTS

The following is a detailed description of embodiments depicted in theaccompanying drawings. The specification is in such detail as to clearlycommunicate the embodiments. However, the amount of detail offered isnot intended to limit the anticipated variations of embodiments. To thecontrary, the intention is to cover all modifications, equivalents, andalternatives consistent with the spirit and scope of the embodiments asdefined by the appended claims.

Wireless communications systems often transmit data using an in-phase(I) and quadrature (Q) format. The I- and Q-channels in an IQ signal arephased-shifted relative to each other by 90 degrees, which is known as aquadrature relationship. The IQ format is popular for data transmissionsbecause an IQ signal is capable of carrying two data streams in thefrequency bandwidth that is normally required by a single data stream.In other words, the IQ format allows twice the data to be sent over agiven frequency bandwidth.

An IQ radio transceiver typically includes separate paths for theI-channel and the Q-channel, both in the transmitter and receiver. Forinstance, after the signal is received in a single antenna, the signalis split into two separate channels, where the I-channel receiver pathcan include a first set of mixers, amplifiers, filters, etc. todown-convert and process the I-channel data. Likewise, the Q-channelreceiver path can include a second set of mixers, amplifiers, filters,etc. to down-convert and process the Q-channel data.

Quadrature imbalance in the radio receiver or transmitter can impair theability to successfully receive or transmit high speed data carried bythe wireless signal. Quadrature imbalance may occur when the I-channelgain is different from that of the Q-channel, or when the phaserelationship between the two channels is not exactly 90 degrees. Inother words, quadrature imbalance is caused by gain and/or phasemismatches of the high frequency components in the I- and Q-channels ofthe IQ transceiver. For example, the receiver components in theI-channel can have slightly different amplitude and/or phasecharacteristics than the receiver components in the Q channel,introducing imbalance or mismatch errors in the I- and Q-basebandsignals. Although the differences are usually small, these gain andphase imbalances reduce the effective signal-to-noise ratio of the IQreceiver, and increase the number of bit errors for a given data rate.

The state-of-the-art in low-cost, low-power wireless transceivers todayis the direct conversion architecture. The direct conversionarchitecture is also susceptible to quadrature imbalance. Due to the useof two physically separate baseband branches, and the generation ofhigh-frequency quadrature signals (0° and 90°), the accuracy of thetransmitted signal and the ability to receive accurately are limited bythe degree of quadrature imbalance. Quadrature imbalance limits theError Vector Magnitude (EVM) of the transceiver, which is especiallycritical in multiple-input and multiple-output (MIMO) systems.

Mass-produced radio frequency integrated circuit (RFIC) systems areusually manufactured in silicon using complementarymetal-oxide-semiconductor (CMOS) processes. Variations in the CMOSmanufacturing processes contribute greatly to the problem of quadratureimbalance. Transceivers may employ calibration to counter or minimizethe effects of quadrature imbalance.

Electronic devices like personal computers, cellular telephones, andpersonal digital assistants (PDAs) may employ direct conversionreceivers to communicate with Wireless Personal Area Networks (WPANs)and Wireless Local Area Networks (WLANs). Additionally, network deviceslike Wireless Access Points (WAPs) and network routers may also employdirect conversion receivers and direct conversion transmitters tocommunicate with other devices in the network. The embodiments hereinmay serve to address quadrature imbalance problems in numerous types ofdirect conversion transceivers, including transceivers in the electronicdevices noted above.

Generally speaking, methods, apparatuses, and systems that calibratequadrature imbalance in direct conversion transceivers are contemplated.An example system embodiment may be in a mobile computing device withwireless communications capabilities, such as an integrated wirelessnetworking card. The card of the mobile computing device may have adirect conversion transceiver configured to communicate with a varietyof wireless networking devices.

During a power-on sequence of the wireless networking card, such as whenthe card is inserted into the mobile computing device, the networkingcard may perform a self-calibration to address quadrature imbalance. Thenetworking card may isolate the transmitter and receiver of the cardfrom any antennas, couple the radio frequency (RF) section of thetransmitter to the RF section of the receiver via a loopback path, andinject a calibration signal into the transmitter. In the loopback path,the networking card may shift the phase of the signal that propagatesthrough the transmitter using two different phase angles to produce twodifferent signals that exit the RF section of the receiver. By measuringthe two different signals that exit the receiver, the network card maybe able to calculate correction coefficients, or parameters, which maybe used to adjust elements that address or correct the quadratureimbalance for both the transmitter and receiver.

A method embodiment may involve a wireless networking station or othercommunication device which employs a direct conversion transceiver andperforms a calibration to correct quadrature imbalance. Thecommunication device may start by injecting a single-frequency signalinto a transmitter of the transceiver to produce a transmitter signal inthe RF portion of the transmitter. The transmitter signal, produced bythe propagation of the calibration signal through the transmitter, mayhave quadrature imbalance due to a mismatch of elements in thetransmitter.

The communication device may continue by generating a firstphase-shifted signal via the transmitter signal and coupling the firstphase-shifted signal to the RF portion of a receiver of the transceiver.For example, the communication device may receive the transmitter signalwhich exits the RF portion of the transmitter and shift the signal by afirst phase angle by circuitry in the loopback path. The communicationdevice may continue by storing a first set of parameters of a firstreceiver signal generated by the first phase-shifted signal. Then thecommunication device may generate a second phase-shifted signal from thetransmitter signal using a second phase angle. In other words, the phaseangles of the first and second phase-shifted signals may differ, such asone phase-shifting angle being +45° while the other is −45°.

The communication device may store a second set of parameters of asecond receiver signal generated by the second phase-shifted signal.Using the first and second set of parameters, the communication devicemay calculate correction parameters for quadrature imbalance.

An embodiment of an apparatus comprises a signal generator to generate asingle-frequency signal to calibrate for quadrature imbalance in adirect conversion transceiver. For example, the single-frequency signalmay generate a transmitter signal from the RF section of a transmitterof the transceiver. The apparatus also comprises a phase-shifting moduleto receive the transmitter signal and to generate a first phase-shiftedsignal and a second phase-shifted signal derived from the transmittersignal. The phase-shifting module may be arranged to couple the firstand second phase-shifted signals to the RF section of a receiver of thetransceiver.

The embodiment of the apparatus comprises a calculation module tocalculate, via a first and second set of parameters, correctionparameters for correction of the quadrature imbalance. The first set ofparameters comprises measurements of a first receiver signal and thesecond set of parameters comprises measurements of a second receiversignal. Generation of the first receiver signal is via the firstphase-shifted signal and generation of the second receiver signal is viathe second phase-shifted signal.

An alternative system embodiment may comprise a cellular telephone orother communication device employing a direct conversion transceivercoupled to an antenna. The transceiver comprises a direct conversiontransmitter and a direct conversion receiver. A phase-shifting module ofthe system may receive a transmitter signal from the RF section of thetransmitter and generate a first phase-shifted signal and a secondphase-shifted signal based on the transmitter signal. In the systemembodiment, generation of the transmitter signal is via injection of asingle-frequency signal into the transmitter.

The system further includes a calculation module coupled to thereceiver. The calculation module is arranged to calculate correctionparameters for correction of quadrature imbalance in the transceiver.The calculation module may calculate the correction parameters via afirst set of parameters and via a second set of parameters derived frommeasurement of receiver signals. The receiver signals include a firstreceiver signal based on the first phase-shifted signal and a secondreceiver signal based on the second phase-shifted signal.

Various embodiments disclosed herein may be used in a variety ofapplications. Some embodiments may be used in conjunction with variousdevices and systems, for example, a transmitter, a receiver, atransceiver, a transmitter-receiver, a wireless communication station, awireless communication device, a wireless Access Point (AP), a modem, awireless modem, a Personal Computer (PC), a desktop computer, a mobilecomputer, a laptop computer, a notebook computer, a tablet computer, aserver computer, a handheld computer, a handheld device, a PersonalDigital Assistant (PDA) device, a handheld PDA device, a network, awireless network, a Local Area Network (LAN), a Wireless LAN (WLAN), aMetropolitan Area Network (MAN), a Wireless MAN (WMAN), a Wide AreaNetwork (WAN), a Wireless WAN (WWAN), devices and/or networks operatingin accordance with existing IEEE 802.16e, 802.20, 3 GPP Long TermEvolution (LTE) etc. and/or future versions and/or derivatives and/orLong Term Evolution (LTE) of the above standards, a Personal AreaNetwork (PAN), a Wireless PAN (WPAN), units and/or devices which arepart of the above WLAN and/or PAN and/or WPAN networks, one way and/ortwo-way radio communication systems, cellular radio-telephonecommunication systems, a cellular telephone, a wireless telephone, aPersonal Communication Systems (PCS) device, a PDA device whichincorporates a wireless communication device, a Multiple Input MultipleOutput (MIMO) transceiver or device, a Single Input Multiple Output(SIMO) transceiver or device, a Multiple Input Single Output (MISO)transceiver or device, a Multi Receiver Chain (MRC) transceiver ordevice, a transceiver or device having “smart antenna” technology ormultiple antenna technology, or the like.

Some embodiments may be used in conjunction with one or more types ofwireless communication signals and/or systems, for example, RadioFrequency (RF), Infra Red (IR), Frequency-Division Multiplexing (FDM),Orthogonal FDM (OFDM), Orthogonal Frequency-Division Multiple Access(OFDMA), Time-Division Multiplexing (TDM), Time-Division Multiple Access(TDMA), Extended TDMA (E-TDMA), Code-Division Multiple Access (CDMA),Multi-Carrier Modulation (MDM), Discrete Multi-Tone (DMT), Bluetooth®,ZigBee™, or the like. Embodiments may be used in various otherapparatuses, devices, systems and/or networks.

Turning now to the drawings, FIG. 1 depicts a direct conversiontransceiver 100 having a transmitter 122 and a receiver 128. Receiver128 is a direct conversion receiver having an amplifier 126. Mixer 130,low-pass filter 142, and analog-to-digital converter (ADC) 144 comprisean I-channel path, while mixer 132, low-pass filter 138, and ADC 140comprise a Q-channel path. As people possessing ordinary skill in theart will appreciate, numerous elements of transceiver 100 have beenomitted from FIG. 1, for the sake of simplicity and ease inunderstanding. For example, local oscillators at the mixer stages,multiplexers, and digital signal processing (DSP) elements coupled tothe ADCs and digital-to-analog converters (DACs) are just some of theelements not depicted in FIG. 1.

Receiver 128 receives an IQ signal at the input to amplifier 126, suchas via an antenna coupled to the input. In other words, an antennareceives an IQ signal over the air and transfers the signal to amplifier126 during operation of receiver 128. Receiver 128 then directlydown-converts the IQ signal to baseband, producing I-channel data at theoutput of ADC 144 and Q-channel data at the output of ADC 140.

The IQ signal carries I-channel data and Q-channel data in the samefrequency bandwidth that would normally be needed for a single datastream. The I-channel data and the Q-channel data may be two distinctdata streams, or the I- and Q-channels may be interleaved to create asingle data stream that occupies only half the normal bandwidth. The IQsignal received by the antenna may be carried on an RF carrier, or someother high frequency carrier suitable for over-the-air transmission. Forinstance, the IQ signal can be a terrestrial or satellite television(TV) signal, or some other type of communications signal, including adata communications signal. In some situations, the input to amplifier126 may be coupled to a cable instead of an antenna, such as that usedin a cable TV system.

The I-channel portion of receiver 128 down-converts and digitizes aportion of the IQ signal, producing an I-baseband signal. Similarly, theQ-channel portion of receiver 128 down-converts and digitizes a portionof the IQ signal, producing a Q-baseband signal. A DSP (not shown)receives the I- and Q-baseband signals from the ADCs and demodulates theI- and Q-baseband signals to process and retrieve the basebandinformation. In observing the operation of receiver 128, one mayappreciate that receiver 128 has an analog section 134 and a digitalsection 136.

Transmitter 122 may operate in a similar fashion but in a reverse mannerto transmit data. Transmitter 122 is a direct conversion transmitterhaving an I-channel path and a Q-channel path. The I-channel path oftransmitter 122 comprises DAC 102, low pass filter 106, and mixer 114.The Q-channel path comprises DAC 110, low-pass filter 112, and mixer116. Also similar to receiver 128, transmitter 122 has an analog section108 and a digital section 104.

The I-channel portion of transmitter 122 takes the I-baseband signal,converts the signal from digital to analog, and up-converts theI-portion of the IQ signal. Similarly, the Q-channel portion oftransmitter 122 takes the Q-baseband signal, converts the signal fromdigital to analog, and up-converts the Q-portion of the IQ signal.Transmitter 122 then mixes and combines the two signals, amplifies theIQ signal via amplifier 120, and transmits the amplified signal, such asby way of an antenna coupled to the output of amplifier 120.

Any gain or phase mismatches between the I path and the Q path maycreate quadrature imbalance. Quadrature imbalance in either transmitter122 or receiver 128 can impact the performance of transceiver 100. Forexample, quadrature imbalance in receiver 128 may reduce the overallsignal-to-noise ratio below an acceptable level and increase the biterror rate. In a more specific example, the mixer 130 may have adifferent amplitude and/or phase characteristic than mixer 132. Thedifferences between the mixers will increase the bit error rate in theresulting I- and Q-baseband signals during demodulation. Similarperformance impact may result from other component mismatches, such asgain and/or phase mismatches between the filters 142 and 138, or theADCs 144 and 140.

FIG. 1 further illustrates a method of calibration that one may employto alleviate the problem of quadrature imbalance in transceiver 100.Transceiver 100 has a phase-shifting module 124 in an added path, calleda loopback path. The loopback path directs the transmitted signal fromthe output of mixers 114 and 116 into analog section 134 of receiver128. The added loopback path of transceiver 100 includes a fixedphase-shift of an arbitrary phase, here 45°. Other transceivers mayinclude other fixed phase-shift values. For example, other transceiversmay use a different arbitrary phase-shift, wherein that phase-shift doesnot equal a multiple of 90°.

Quadrature imbalance may be treated as a complex-plane operation whichcan be represented in matrix form:

${\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix} = {\begin{bmatrix}\alpha & \beta \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}I \\Q\end{bmatrix}}};{\alpha \sim 1};{{\beta }{\operatorname{<<}1}}$

In the matrix representation, α is the gain imbalance factor and β isthe phase imbalance factor. For a transceiver using RF loopback with aphase shift, such loopback is equivalent to multiplying three matricesin order:

${\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix} = {\begin{bmatrix}\alpha_{RX} & \beta_{RX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}{\cos \left( \phi_{LB} \right)} & {\sin \left( \phi_{LB} \right)} \\{- {\sin \left( \phi_{LB} \right)}} & {\cos \left( \phi_{LB} \right)}\end{bmatrix} \cdot \begin{bmatrix}\alpha_{TX} & \beta_{TX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}I \\Q\end{bmatrix}}};$ N ⋅ 90^(^(∘)) ≠ ϕ_(LB) ≠ 0^(^(∘))

In the matrices above, α_(RX) and α_(TX) are the gain imbalance factorsfor receiver 128 and transmitter 122, respectively. β_(RX) and β_(TX)are the phase imbalance factors for receiver 128 and transmitter 122,respectively. φ_(LB) is the loopback phase-shift.

Transceiver 100 may employ a method for measuring the matrixcoefficients. For example, transceiver 100 may use correlation in thedigital domain to determine I′ and Q′. Further, transceiver 100 knowsthe matrix coefficients of the transmitted signal, I and Q, and knowsthe amount of loopback phase shift (φ_(LB)=45° in the example of FIG.1). Transceiver 100 may use four equations to solve for four variables.Using four equations to solve for the four unknown variables enables thesystem to be analytically solved. That is to say, transceiver 100possesses enough information to solve for α_(RX), α_(TX), β_(RX) andβ_(TX).

Various embodiments may extend the aforementioned methodology to solvefor α_(RX), α_(TX), β_(RX), and β_(TX). The embodiments may furtherenable solving for α_(RX), α_(TX), β_(RX), and β_(TX) without requiringexact knowledge of the phase shift. Because many transceiver systems maycomprise practical, mass-produced, transceiver circuits with all theaccompanying circuit variations due to component tolerances, knowingexactly the phase shift may generally pose a serious challenge.

FIG. 2 illustrates how a direct conversion transceiver 200 may perform acalibration to alleviate quadrature imbalance according to oneembodiment. Transceiver 200 may perform a calibration to alleviatequadrature imbalance without precisely knowing the phase shift anglesused during the calibration process. By inspection, one may note thattransceiver 200 in FIG. 2 is similar in many respects to transceiver 100in FIG. 1.

Transceiver 200 also comprises a direct conversion transmitter 222 and adirect conversion receiver 228. Mixers 230 and 232, low-pass filters 242and 238, and ADCs 244 and 240 comprise the I-channel path and Q-channelpaths of receiver 228, respectively, as described in the discussion forFIG. 1. Similarly, transmitter 222 has an I-channel path and a Q-channelpath. The I-channel path of transmitter 222 comprises DAC 202, low passfilter 206, and mixer 214. The Q-channel path of transmitter 222comprises DAC 210, low-pass filter 212, and mixer 216. Receiver 228 hasa digital section 236 and an analog section 234, while transmitter 222has an analog section 208 and a digital section 204. As with transceiver100, the components in the I- and Q-channel paths of receiver 228 andtransmitter 222 may be sources of quadrature imbalance.

Additionally, transceiver 200 has a quadrature calibration controller250 in the digital section and a different phase-shifting module 224 inthe RF section. Calibration controller 250 may direct or control thecalibration process, manipulating phase-shifting module 224 to injectsignals having different phase shift angles into receiver 228.

Transceiver 200 comprises a signal generator 252 to generate a cleansingle-frequency calibration signal. For the embodiment depicted in FIG.1, signal generator 252 comprises an internal digital frequency source.In alternative embodiments, the signal generator may comprise an analogsource. For example, in one alternative embodiment, an analog signalgenerator module may inject a calibration signal into the I- andQ-channel paths between the DACs (202 & 210) and the low-pass filters(206 & 212). In other words, the alternative embodiment may ignore anygain and/or phase mismatches associated with DAC 202 and DAC 210 andonly correct the effects of quadrature imbalance associated with theremaining components in the l- and Q-channel paths.

As noted, transceiver 200 also comprises phase-shifting module 224 in aloopback path which couples analog section 208 of transceiver 222 toanalog section 234 of receiver 228 during calibration. That is to say,during normal operation, phase-shifting module 224 may be isolated fromanalog sections 208 and 234. During calibration, however, calibrationcontroller 250 may couple phase-shifting module 224 to analog sections208 and 234 by closing solid state switches, switching betweenmultiplexer inputs, or activating other switching devices.

During calibration of transmitter 222 and receiver 228, calibrationcontroller 250 may activate signal generator 252 and enable signalgenerator 252 to inject the single-frequency signal into DAC 202 and DAC210. Upon exiting the digital section 204, the calibration signal maypropagate through low-pass filters 206 and 212, as well as mixers 214and 216. In propagating through the DACs, the filters, and the mixers,the component mismatches may cause a quadrature imbalance for thecalibration signal, resulting in an altered signal at node 218. Thealtered signal at node 218 may be referred to as the transmitter signal.

Calibration controller 250 may cause phase-shifting module 224 togenerate a first phase-shifted signal based on the transmitter signal.In other words, the transmitter signal may propagate from node 218 intophase-shifting module 224. Phase-shifting module 224 may shift the phaseof the transmitter signal by a first phase-shift angle (Φ₁) to producethe first phase-shifted signal. The first phase-shifted signal may enterthe RF portion of receiver 228 via the loopback path, at the input tomixers 230 and 232. The first phase-shifted signal may propagate throughthe I- and Q-channel elements of receiver 228, producing a firstreceiver signal at the outputs of ADC 244 and ADC 240.

As the first receiver signal moves from analog section 234 into digitalsection 236 by exiting the ADCs, calibration controller 250 may measurea first set of parameters via measurement module 260. Measurement model260 may take numerous samples of the digital values produced by ADC 244and ADC 240. From the numerous samples, calculation module 256 may beable to determine the magnitude of the first receiver signal. In otherwords, calculation module 256 may be able to determine I′ and Q′ for thefirst receiver signal. Calibration controller 250 may store the firstset of parameters for the first receiver signal into memory module 258.

Further, measurement module 260 may also take numerous samples of thedigital values produced by signal generator 252 during the calibrationprocess. Stated differently, calculation module 256 may be able todetermine I′ and Q′ for the first receiver signal, as well as haveknowledge of the I and Q parameters that were used during the creationand measurement of the I′ and Q′ parameters. Worth noting, alternativeembodiments may not necessarily use measurement module 260 to obtain theI and Q parameters. For example, it may not be necessary to measure thevalues if the calibration signal is fixed and will not vary. Such I andQ parameters may comprise fixed, stored parameters in memory module 258or calculation module 256.

Calibration controller 250 may then cause phase-shifting module 224 togenerate a second phase-shifted signal based on the transmitter signal.That is to say, phase-shifting module 224 may switch to a second mode ofoperation and shift the phase of the transmitter signal by a secondphase-shift angle (Φ₂), producing a second phase-shifted signal.Calibration controller 250 may measure a second set of parameters atmeasurement module 260 related to the second phase-shifted signal. Fromthe numerous samples, calculation module 256 may be able to determinethe magnitude of the second receiver signal. Stated differently,calculation module 256 may be able to determine I′ and Q′ for the secondreceiver signal. Calibration controller 250 may store the second set ofparameters for the second receiver signal into memory module 258.

Calibration controller 250 may then use the first and second set ofstored parameters in memory module 258 to calculate correctionparameters. The calculated correction parameters may enable calibrationcontroller 250 to calibrate transmitter 222 and receiver 228 forquadrature imbalance via the first and second set of parameters. Forexample, calibration controller 250 may comprise internal digitalelements in correction module 254 that enable correction of quadratureerrors in transmitter 222 and receiver 228. In alternative embodiments,a transceiver may calibrate the transmitter and/or receiver in adifferent manner. For example, instead of employing solely digitalelements in the digital section, calibration controller 250 may insteaduse the calculated correction parameters to adjust elements in theanalog sections, such as phase compensation components and gaincompensation components.

The embodiment depicted in FIG. 2 employs digital correction for phaseimbalance and gain imbalance correction. Once the correction parametersare calculated for transmitter 222, correction module 254 may take theone or more of the digital values that would otherwise be transferred toDAC 202 or 210 without correction, adjust the digital value(s) based onthe correction parameters, and transfer the adjusted digital value(s) toDAC 202 and/or 210. Correction module 254 may operate in a similar butreverse manner for receiving digital signals from ADC 244 and 240.

An alternative embodiment of a correction module 300 is depicted in FIG.3. Instead of being employed in the digital portion of a transceiver,correction module 300 may correct gain imbalance and phase imbalanceerrors in analog section 208 or 234. For example, correction module 300comprises a phase compensation module 310 and a gain compensation module320. Correction module 300 may be inserted between low-pass filters(206, 212; 242, 238) and the DAC or ADC elements (202, 210; 244, 240).Based on the calculated correction parameters, correction module 300 mayadjust the phase or gain of the signal transferred to the low-passfilters or to the DAC/ADC elements.

FIG. 2 and the associated discussion illustrate how calibrationcontroller 250 and phase-shifting module 224 may operate in together tocreate a new set of equations to solve:

${\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix} = {A_{1} \cdot \begin{bmatrix}\alpha_{RX} & \beta_{RX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}{\cos \left( \phi_{{LB},1} \right)} & {\sin \left( \phi_{{LB},1} \right)} \\{- {\sin \left( \phi_{{LB},1} \right)}} & {\cos \left( \phi_{{LB},1} \right)}\end{bmatrix} \cdot \begin{bmatrix}\alpha_{TX} & \beta_{TX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}I \\Q\end{bmatrix}}},{{{and}\begin{bmatrix}I^{\prime} \\Q^{\prime}\end{bmatrix}} = {A_{2} \cdot \begin{bmatrix}\alpha_{RX} & \beta_{RX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}{\cos \left( \phi_{{LB},2} \right)} & {\sin \left( \phi_{{LB},2} \right)} \\{- {\sin \left( \phi_{{LB},2} \right)}} & {\cos \left( \phi_{{LB},2} \right)}\end{bmatrix} \cdot \begin{bmatrix}\alpha_{TX} & \beta_{TX} \\0 & 1\end{bmatrix} \cdot \begin{bmatrix}I \\Q\end{bmatrix}}},{{{with}\mspace{14mu} \phi_{{LB},1}} \neq {\phi_{{LB},2}.}}$

Allowing, without loss of generality, the two phase-shift states (onestate based on φ_(LB,1) and one state based on φ_(LB,2)) to havedifferent gain levels, one may observe that eight equations remain to besolved using eight measurements. The eight equations to solve are thetwo gain imbalance parameters (α_(RX) and α_(TX)), the two phaseimbalance parameters (β_(RX) and β_(TX)), the two loopback phases(φ_(LB,1) and φ_(LB,2)), and the two overall gains (A₁ and A₂) usingeight measurements (two I′ parameters, two Q′ parameters, two Iparameters, and two Q parameters for the two states). The equations forthe two states are analytically solvable, and no knowledge of theloopback phases or the gains is needed to solve the equations anddetermine the values of α_(RX), α_(TX), β_(RX), and β_(TX).

In many embodiments the switchable-phase element, phase-shifting module224, may be implemented using a Poly-Phase Filter (PPF). The PPF mayprovide two differential outputs with a 90° phase offset between thetwo. Each stage of the PPF may add 45° to the overall phase shift of thefilter. So, by choosing either one differential output or the other (foran odd number of stages) or by choosing to add or subtract the twooutput signals (for an even number of stages) one may generate the twophases reliably using mostly passive elements. Because phase-shiftingmodule 224 may perform the switch in the RF sections, the switching maynot generally impact the quadrature imbalance of either the transmitteror the receiver.

FIG. 4 depicts one embodiment of an apparatus 400 that may calibratetransmitter and/or receivers in a direct conversion transceiver 480. Forexample, apparatus 400 may comprise, or at least form a part of, awireless network communication device, such as a wireless access pointdevice. With reference to FIG. 2, controller 410 may correspond tocalibration controller 250, while phase-shifting module 470 maycorrespond to phase-shifting module 224. One or more elements ofapparatus 400 may be in the form of hardware, software, or a combinationof both hardware and software. For example, in the embodiment depictedin FIG. 4, one or more portions of calculation module 440 may compriseinstruction-coded modules stored in one or more memory devices. Forexample, the modules may comprise software or firmware instructions ofan application in a DSP that employs a microprocessor for performingcomplex calculations.

In alternative embodiments, one or more of the modules of apparatus 400may comprise hardware-only modules. For example, signal generator 420,measurement module 430, calculation module 440, receiver correctionmodule 450, transmitter correction module 460, and phase-shifting module470 may each comprise a portion of an integrated circuit chip. In suchembodiments, one or more of the modules may comprise such hardwareelements as resistors, capacitors, inductors, diodes, CMOS transistors,analog transistors, combinations of logic gates, and state machines.

In even further alternative embodiments, one or more of the modules ofapparatus 400 may comprise a combination of hardware and softwaremodules. For example, controller 410 may comprise firmware-encodedinstructions executed by a processor, when the processor works inconjunction with a state machine which directs the overall calibrationprocess and controls the various elements, and performs calculations todetermine the values of the correction parameters after the measurementof the sets of parameters have been stored by measurement module 430.

Signal generator 420 may generate the calibration signal to be injectedinto the receiver. As noted previously, signal generator 420 may be adigital signal generator in some embodiments or an analog signalgenerator in other embodiments. In many embodiments, signal generator420 may comprise a fixed signal generator. However, in otherembodiments, signal generator 420 comprise a variable signal generator,wherein controller 410 may be able to select a particular calibrationsignal frequency or calibration signal amplitude.

In many embodiments, measurement module 430 may comprise buffers orseries of memory elements coupled to a status register associated withan ADC. When triggered by controller 410, measurement module 430 may beconfigured to store a series of values retrieved from the statusregister during a cycle of the received signal and store the values inthe memory elements for later retrieval for the subsequent calculations.In many embodiments, measurement module 430 may comprise dynamic randomaccess memory (DRAM) to store the measured sets of parameters. In someembodiments, measurement module 430 may employ another type of memory tostore the measured sets of parameters, such as static RAM or flashmemory.

As noted, calculation module 440 may retrieve the first and second setsof parameters and use them to determine the values for the correctionparameters. In many embodiments, calculation module 440 may comprise astate machine. In alternative embodiments, calculation module 440 maycomprise a dedicated processor, such as a microcontroller of anapplication specific integrated circuit (ASIC) coupled with controller410.

In the embodiment of FIG. 4, apparatus 400 comprises a controller 410that employs two correction modules, 450 and 460. Receiver correctionmodule 450 may be dedicated for calibration of the receiver fortransceiver 480 based on the calculated calibration parameters.Similarly, transmitter correction module 460 may be dedicated forcalibration of the transmitter of correction module 480 based on thecalculated calibration parameters. As noted previously, the correctionmodules may be implemented digitally or in an analog fashion indifferent embodiments.

The number of modules in an embodiment of apparatus 400 may vary. Someembodiments may have fewer modules than those module depicted in FIG. 4.For example, one embodiment may integrate receiver correction module 450and transmitter correction model 460 into a single module. Furtherembodiments may include more modules or elements than the ones shown inFIG. 4. For example, an alternative embodiment may include two or moremeasurement modules, such as an embodiment that employs one measurementper each channel. Other embodiments may include more of the othermodules.

FIG. 5 depicts a flowchart 500 illustrating a method for calibratingquadrature imbalance in direct conversion transceivers. Flowchart 500begins with injecting a single-frequency calibration signal into atransmitter of a direct conversion transceiver (element 510). Forexample, signal generator 252 of FIG. 2 may generate a digitalsingle-frequency calibration signal, comprising I-component values andQ-component values. During a calibration process, calibration controller250 may couple the outputs of signal generator 252 to digital section204 of transmitter 222, enabling signal generator 252 to transfer theI-component values and Q-component values to DAC 202 and DAC 210.

The method according to flowchart 500 also includes coupling thetransceiver RF sections together (element 520), which enables thecalibration signal to propagate through the transmitter and back throughthe receiver. Again referring to FIG. 2, calibration controller 250 mayswitch the states of several transistors, which cause a loopback path tobe created between the analog sections 208 and 234. Upon coupling thetransceiver analog sections together (element 520), an embodimentaccording to flowchart 500 involves generating phase-shifted signalsfrom the transmitter signal (element 530). Continuing with the previousexample, calibration controller 250 may manipulate phase-shifting module224, causing phase-shifting module 224 to shift the phase of thetransmitter signal exiting mixers 214 and 216. Phase-shifting module 224may first shift the phase of the transmitter signal by a first phaseangle of 45°. During a later sequence of the calibration process,calibration controller 250 may manipulate phase-shifting module 224 andcause phase-shifting module 224 to shift the phase of the transmittersignal by a second phase-shift angle of −45°. The 45° and −45°phase-shift angles are only for one embodiment. Other embodiments mayuse two other angles.

The method according to flowchart 500 also includes measuring parametersof receiver signals based on the phase shifted signals (element 540).Again continuing with the example, as the first phase-shifted signalpropagates back through receiver 228, the components of receiver 228will alter the signal and produce a first receiver signal havingslightly altered phase and gain values. Calibration controller 250 mayoperate measurement module 260 to sample and store digitally-sampledvalues for the digital waveform values of the first receiver signal thatADC 244 and ADC 240 transfer to calibration controller 250.

After calibration controller 250 operates or manipulates phase-shiftingmodule 224 to shift the phase of the transmitter signal by the secondangle of −45°, calibration controller 250 may again operate measurementmodule 260 to sample and store digitally-sampled values for the digitalwaveform values of the second receiver signal that ADC 244 and ADC 240transfer to calibration controller 250. Further, and at or about thesame time, calibration controller 250 may operate measurement module 260to sample and store digitally-sampled values for the calibration signalfrom signal generator 252.

The method according to flowchart 500 also comprises calculatingcorrection parameters based on the first and second sets of measuredparameters or sampled values (element 550). Again continuing with ourexample, calculation module 256 may solve eight equations using eightmeasurements. Calculation module 256 may solve for the two gainimbalance parameters (α_(RX) and α_(TX)), the two phase imbalanceparameters (β_(RX) and β_(TX)), the two loopback phases (φ_(LB,1) andφ_(LB,2)), and two overall gains (A₁ and A₂).

Calculation module 256 may solve the eight equations using the eightmeasurements obtained from sampling the two I′ parameters, the two Q′parameters, the two I parameters, and the two Q parameters for the twostates (one state with a phase shift of 45°, the other state with thephase shift of −45°). Calculation module 256 may analytically solve theequations for the two states having no knowledge of the loopback phasesor the gains and determine the values of α_(RX), α_(TX), β_(RX), andβ_(TX).

The method according to flowchart 500 also comprises performingcalibration for quadrature imbalance in the transmitter and receiver(element 560). For example with reference to FIG. 4, controller 410 maycomprise internal digital elements in receiver correction module 450 andtransmitter correction module 460 that enable correction of quadratureerrors in the transmitter and receiver of transceiver 480.

As noted earlier, one or more portions of some embodiments may beimplemented as a program product stored in a tangible medium for usewith a process to perform operations for processes, such as theprocesses described in conjunction with apparatus 400 illustrated inFIG. 4. The program(s) of the program product defines functions of theembodiments (including the methods described herein) and may becontained on a variety of data-bearing media. Illustrative data-bearingmedia include, but are not limited to: (i) information permanentlystored on non-writable storage media (e.g., read-only memory deviceswithin a station); and (ii) alterable information stored on writablestorage media (e.g., flash memory). Such data-bearing media, whencarrying computer-readable instructions that direct the functions ofdevices or systems, represent elements of some embodiments of thepresent invention.

In general, the routines executed to implement the embodiments, may bepart of an operating system or a specific application, component,program, module, object, or sequence of instructions. The computerprogram of an embodiment may be comprised of a multitude of instructionsthat will be translated by a computer into a machine-readable format andhence executable instructions. Also, programs may be comprised ofvariables and data structures that either reside locally to the programor are found in memory or on storage devices. In addition, variousprograms described hereinafter may be identified based upon theapplication for which they are implemented in a specific embodiment ofthe invention. However, it should be appreciated that any particularprogram nomenclature that follows is used merely for convenience, andthus a specific embodiment should not be limited to use solely in anyspecific application identified and/or implied by such nomenclature.

It will be apparent to those skilled in the art having the benefit ofthis disclosure that the embodiments herein contemplate systems,apparatuses, and methods for transmitting responses to transmitted dataframes over wireless networks with varying Interframe Space (IFS) times.It is understood that the form of the embodiments shown and described inthe detailed description and the drawings are to be taken merely asexamples. It is intended that the following claims be interpretedbroadly to embrace all the variations of the embodiments disclosed.

Although some aspects have been described in detail for someembodiments, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims.

Although one embodiment may achieve multiple objectives, not everyembodiment falling within the scope of the attached claims will achieveevery objective. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosure of the embodiments, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe embodiments herein. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1-20. (canceled)
 21. An apparatus, comprising: a signal generator togenerate a signal for calibration to a radio frequency (RF) portion of atransmitter or an RF portion of a receiver of a direct-conversionarchitecture to calibrate for mismatches of in-phase and quadrature(I/Q) signals; at least one of the RF portions coupled by a loopbackpath; and the loopback path to couple first and second signals basedupon the signal generated for calibration to the RF portion of thereceiver, the loopback path comprising a phase-shifting module to shiftthe signal generated for calibration as propagated through a portion ofthe transmitter to generate the second signal with a different phasefrom the first signal; wherein the apparatus is configured to digitallycorrect mismatches of I/Q signals in the receiver and the transmitterbased upon the first and second signals.
 22. The apparatus of claim 21,wherein the apparatus comprises a digital section configured todigitally correct mismatches of I/Q signals.
 23. The apparatus of claim21, wherein the apparatus comprises a calculation module to calculatecorrection parameters based upon the first and second signals.
 24. Theapparatus of claim 21, wherein the apparatus comprises a correctionmodule to digitally correct mismatches of I/Q signals in the receiverand the transmitter based upon the correction parameters.
 25. Theapparatus of claim 21, wherein the signal generator comprises a portionof an integrated circuit chip.
 26. The apparatus of claim 25, whereinthe integrated circuit chip further comprises one or more of ameasurement module, a calculation module, a correction module, and thephase-shifting module.
 27. The apparatus of claim 21, wherein the signalgenerator is configured to generate the signal for calibrationconsisting of a single frequency.
 28. The apparatus of claim 21, whereinthe apparatus is configured to transmit the signal for calibration fromthe transmitter to the receiver via the loopback path.
 29. The apparatusof claim 21, wherein the apparatus is configured to digitally correctmismatches of I/Q signals in the receiver after analog-to-digitalconversion and in the transmitter prior to digital-to-analog conversionbased upon the correction parameters.
 30. The apparatus of claim 21,wherein the apparatus is configured to perform calibration withoutprecisely knowing the different phases.
 31. The apparatus of claim 21,wherein the apparatus improves the Error Vector Magnitude (EVM) of thetransceiver.
 32. The apparatus of claim 21, wherein the apparatuscomprises a multiple-input and multiple-output (MIMO) transceiver. 33.The apparatus of claim 21, wherein the calibration signal is sent fromthe transmitter digital-to-analog converter (DAC).
 34. An integratedcircuit chip, comprising: a signal generator to generate a signal forcalibration to a radio frequency (RF) portion of a transmitter or an RFportion of a receiver of a direct-conversion architecture to calibratefor mismatches of in-phase and quadrature (I/Q) signals; the RF portionof the transmitter; the RF portion of the receiver coupled with the RFportion of the transmitter by a loopback path; the loopback path tocouple first and second signals based upon the signal generated forcalibration to the RF portion of the receiver, the loopback pathcomprising a phase-shifting module to shift the signal generated forcalibration as propagated through a portion of the transmitter togenerate the second signal with a different phase from the first signal;and one or more modules to digitally correct mismatches of I/Q signalsin the receiver and the transmitter based upon the first and secondsignals.
 35. The apparatus of claim 34, wherein the one or more modulescomprise a calculation module and a correction module.
 36. The apparatusof claim 34, wherein the one or more modules comprise a memory module.37. A system, comprising: at least one integrated circuit chip coupledwith an antenna, to generate a signal for calibration to a radiofrequency (RF) portion of a transmitter or an RF portion of a receiverof a direct-conversion architecture to calibrate for mismatches ofin-phase and quadrature (I/Q) signals; the at least one integrated chipcomprising: the RF portions coupled by a loopback path; the loopbackpath to couple first and second signals based upon the signal generatedfor calibration to the RF portion of the receiver, the loopback pathcomprising a phase-shifting module to shift the signal generated forcalibration as propagated through a portion of the transmitter togenerate the second signal with a different phase from the first signal;and one or more modules to digitally correct mismatches of I/Q signalsin the receiver and the transmitter based upon the first and secondsignals.
 38. The apparatus of claim 37, further comprising an amplifierin the RF portion of the transmitter coupled with the antenna.
 39. Theapparatus of claim 37, further comprising an amplifier in the RF portionof the receiver coupled with the antenna.